Everything is in the title :) Just put a dialog saying "To protect your contacts privacy, you should put their addresses in the Cci field instead of To or Cc" with a button "Put addresses from To or Cc to the Cci field" and "I don't care", or something like this. You can also look at https://bugzilla.mozilla.org/show_bug.cgi?id=271405
Personally, I disagree with such addition. That's something the user should already know, if (s)he doesn't, then (s)he should read some stuff. Or are we pointing another situations like this one? Unfortunately this one is not described in rfc1855 (at least I didn't find it), but there should be other resources with extended etiquette and other recommendations for e-mail usage (and if there's none, one can be written). This will also bring up a new dialog, if I understood the idea. It will force the user to hit a few more keystrokes to send the message. If you really want to do it, please add a setting to disable all warnings of this sort.